1. Field of the Invention
The present invention generally relates to capacitive fluid level sensing and, more particularly, to a capacitive fluid level sensing pipette probe for use in an automated chemical analyzer, such as, for example, an automated immunoassay analyzer.
2. Description of the Prior Art
Many chemical analysis processes involve adding a reagent to a sample, allowing a chemical reaction to occur, and then analyzing the sample to determine its constitutes. An immunoassay is a well known type of chemical analysis method used to determine the amount of an analyte in a sample such as plasma or urine. It is based on the interaction of antibodies with antigens, and because of the degree of sensitivity for the analyte (either antigen or antibody), an immunoassay can be used to quantitatively determine very low concentrations of drugs, hormones, polypeptides, or other compounds found in a test sample.
Chemical analyses, such as immunoassays, were historically performed by hand by a trained laboratory technician. Recently, many companies have begun producing automated analyzers. Such systems are computerized and may utilize a conveyer chain or belt to convey sample vessels from station to station whereat specific analysis steps are carried out. The sample vessels may be bar coded to instruct the computer of its contents and the specific test which is to be performed on each sample. Based on this information, a precise volume of fluid is pipetted from a reagent container or a sample vessel to a test tube with a pipette probe.
Depending on the test to be performed, a multitude of reagents may be required. Some tests may even require a combination of several reagents. One way to prevent cross-contamination between samples, has been to use a separate pipette probe for each reagent. This is undesirable since it adds greatly to the mechanical complexity of the analyzer and requires an additional wash step for the pipette tip to reduce the level of contamination between the test samples and unspent reagent supply.
An automated immunoassay system must always generate results which are precise, accurate and independent of one another. These criteria are challenging to achieve since, typically laboratory workloads may demand that an automated system generate as many as 120 test results per hour. Although all instrument processes contribute to the instrument's ability to produce such results, perhaps none is more important than the pipetting process. As such, it is imperative that the pipette probe be able to transfer liquids precisely and accurately, with virtually no sample-to-sample carryover. It is also important to be able to sense when a pipette probe has just contacted a fluid surface. For container tubes containing only a small amount of liquid it is imperative that the pipette probe stop prior to the tip of the probe crashing into the bottom of the tube.
One way to minimize cross contamination and prevent bottom crashes is to accurately detect the fluid level in a tube containing a sample or a reagent. U.S. Pat. No. 4,897,244 to Wallace et al. is directed to a level sensing arrangement for sensing the descent of a probe into a fluid. A container containing a fluid sample rests on a flat, grounded surface. A probe is connected to an AC signal source. A cable connects the probe to a remote impedance sensing circuit. When the probe is lowered and contacts the fluid sample, the remote impedance sensing circuit senses an increase in capacitance. A control circuit uses the impedance information to operate a mechanical drive. The impedance measurements are used to determine the depth of the probe in the container.
As discussed above, a capacitive level sensing circuit must be able to accurately detect when the tip of the probe has just touched the fluid surface. Unfortunately, the above described circuit suffers from some drawbacks. Referring to FIG. 1, while the sensing element is the probe 2 itself, the sensing circuitry 4 is located elsewhere and is connected to the probe by a coaxial cable 6. The nature of the co-axial cable allows the cable to bend and flex. This is understandable since the sensing circuitry 4 is stationary and the probe 2 is mobile, that is, its function is to repeatedly raise and lower into a vessel 8 containing fluid. The coaxial cable on available probes is upwards of six inches long and itself has a capacitance upwards of 20 pf. Additionally, the flexing of the cable can cause a change in capacitance of up to 1 pf, unrelated to the position of the probe relative to the fluid.
Since the quantifies of fluid aspirated by the probe is extremely small, often in the order of microliters, the capacitance change when the probe tip contacts the fluid may often be less than 1 pf. Hence, errors will occur since the control circuitry will mistake the capacitance change due to the flexing of the cable for the expected change due to the probe tip contacting the fluid.